No, it does not involve a naughty Smurf or the moon turning strange colors. It simply means there will be two full moons during the month of December. The first was Dec. 2.

The monthly calendar is roughly based on a lunar cycle, said Syracuse University
physics professor, Peter Saulson, who researches astrophysics. The lunar cycle is about 29 days, he said, and the calendar month is a day or two longer. So a few days build up each month that, eventually, mean a full moon will rise in the beginning of a month and also at the end of the month. And this is one of those months.

Blue moons occur about every 2.5 years. The last blue moon was May 31, 2007.

This particular blue moon may seem unique because it falls on the last day of the year. But really, Saulson said, it's not that unusual. Blue moons are always going to fall at the end of the month, he said. The last December blue moon was less than 20 years ago: Dec. 31, 1990, also a New Year's Eve blue moon, he said. The next one won't come again until 2028.

The New Year's Eve blue moon will be visible in the United States, Canada, Europe, South America and Africa. For partygoers in Australia and Asia, the full moon does not show up until New Year's Day, making January a blue moon month for them.

However, the Eastern Hemisphere can celebrate with a partial lunar eclipse on New Year's Eve when part of the moon enters the Earth's shadow. The eclipse will not be visible in the Americas.

The modern definition of a blue moon is actually an error, Saulson said. Blue moons used to refer to the seasons that had four full moons instead of the usual three. The third of four moons in the season was called the "blue moon" so that the last moon of the season could still be called the "late moon." A 1940s error in the interpretation of the definition led to the modern-day definition of a blue moon being the extra full moon in a month.

The last seasonal blue moon was in May 20, 2008.

Blue Moon Brewing Co., which got its name from the saying "once in a blue moon," is commemorating the New Year's Eve blue moon with a limited-quantity beer called Grand Cru. The Colorado-based beer company is also having a world-wide midnight toast on Facebook.

Saulson said he doesn't know of any sky-watching buffs who are planning anything special to celebrate the moon. After all, he said, it's not going to look any different than any other full moon. But maybe it's a sign of good luck, he said.